The technique of using porous materials as spargers to aerate fluids, such as the addition of carbon dioxide to beverages or the addition of oxygen to blood for medical purposes, is known in the art. It is known to consolidate materials such as refractory particles or thermoplastic spheres or sintered, powdered metal to form a sparger suitable for aeration of liquids.
Prior art spargers have had problems and disadvantages associated at least in part with the materials used. When a sparger is to be used for medical purposes, uniform porosity, sterilizability and lack of toxicity are important. Toxicity can be a problem when the sparger material or binder, e.g., metallic or resinous materials, or components thereof, dissolve in or cause changes in the medical solution or body fluids being aerated.
It is also difficult to obtain uniform porosity throughout a sparger incorporating thermoplastic materials due to the plastic nature of the material, especially when the material is warm pressed under pressure. Warm pressing of thermoplastic spheres generally results in a distortion of the spheres. The resulting nonuniformly distributed porosity causes nonuniform bubbles during use of the spargers. Blood spargers present an especially difficult problem since many substances, although inert, will cause reactions in the blood or cause clotting, e.g., glass initiates clotting when in contact with blood for a time.
Spargers made of refractory ceramics or sintered powder metal have good temperature resistance; however, many of the metallic materials corrode or could result in toxic products in liquids aerated for medical purposes. Additionally, the cost of forming refractory or powdered metal spargers is considerably greater than when less expensive materials, such as glass, are used due to the higher cost of metal and refractory powders and the higher temperatures and increased costs necessary to properly sinter refractory or metal powders together. It has also been difficult to form refractory or powdered metal materials which have uniform porosity, especially where the spargers are a complex shape, since pressing of the metal or refractory powders results in differential die forces within the pressing cavity causing a nonuniform porosity in the finished article.
U.S. Pat. No. 3,166,615 describes a method for forming porous rigid structures in which solid particles are coated with a very thin film of fluid thermosetting resin and before the resin hardens, a structure is formed by pressing a mass of such particles into contact with each other displacing the resin film from the point of contact without filling the interstices between the particles. The resin is then cured. It appears that under these circumstances, the resin being in a fluid or perhaps glassy state, draws up into a meniscus between the particles near the points of contact. Little, if any, resin appears to remain over the surface of the particle. The result is that the body thus formed probably depends for its strength in tension or bending stress mainly upon the bond between the particle surface and the resin. All the particles are said to be in direct contact with each other throughout the porous mass thus formed.